In the manufacture of semiconductor devices, silicon wafers, for example, are oxidized to provide a thin layer of oxide 5,000 to 10,000 A.degree. thick. In order to remove this oxide layer in preselected areas for the dopant diffusion step which produces junctions, a photoresist material is applied to the oxidized wafer surface and subsequently exposed to UV radiation through a suitable mask defining the preselected areas. Depending on whether a positive or negative working photoresist is employed, the exposed or unexposed regions respectively are removed upon subsequent development in a suitable solvent to expose the underlying oxide. A well known method of removing the oxide layer in the exposed areas includes a wet chemical acid etch dip, the resist serving as an acid barrier in the areas on which it remains. Further processing requires a rinsing and drying step, removal of remaining photoresist with a solvent and a further acid rinse to rid the wafer surface of inorganic residues.
More recently, radio frequency (RF) generated plasma devices have been employed to carry out the etching of oxide layers or semiconductor wafers as well as the complete removal of photoresist materials by a procedure known as ashing. With these devices gases and the workpieces within a glass or quartz reaction chamber are inductively coupled to an external RF coil operating usually at 13.56 MHz at power levels of 50 to 300 watts. Under such conditions, the plasma is designated as "cold" and the temperatures within the reaction chamber due to the imposition of the plasma will normally not exceed a maximum of 300.degree. to 350.degree. C. Temperature ranges between approximately 100.degree. and 350.degree. C. will usually be attained in various locations inside the plasma chamber and generally in non-uniform fashion, depending, to some extent, on the size, volume, and location of the workpieces inserted in the plasma area. It has been found that ashing of most photoresists leaves objectionable residues which must be removed in a subsequent processing step. Gaseous plasma etching of SiO.sub.2 films may be effected by ionizing certain organo halides in a suitable reactor chamber. Low temperature oxidation of the photoresist is achieved by subjecting it to active oxygen species generated at RF frequencies in a reactor. Even residual inorganic residues from the photoresist may be simultaneously removed by utilizing mixtures of halogen containing gases with oxygen during the ashing procedure. This technique avoids the problem of superficially oxidizing the previously bare silicon surface during normal oxygen plasma ashing procedures.
There are a number of operational problems associated with RF plasma reactions. First of all while the RF power effectively couples to the reactive gas, it also couples to the workpiece at the same time and the system provides no independent means of controlling the temperature of the target or workpiece. Second, there is a non-uniformity of RF coupling to the workpiece and consequently variable rates of chemical reaction with the reactive gas. Furthermore, while the RF generated plasma more or less completely fills the reaction chamber, its concentration and therefore its temperature varies from point to point from target to the chamber wall, contributing again to non-uniform reactivity with the target. The conditions of operation including size, shape, volume, and location of the workpieces must be rigidly controlled since even slight variations from the specified condition will cause the results obtained to vary to an intolerable extent in the precision product. Thus the flexibility of RF plasma operation for development and etching of semiconductors is severly limited, since any change in an operational parameter requires an exhaustive step-by-step tuning process, thus imposing a severe limitation in productivity. In addition, because the plasma extends to the wall of the chamber, it can react with the wall and supply impurities to the specimen depending on the nature of the wall and of the gaseous plasma. For example, when halogen and in particular, fluorine containing etch gases are employed, severe etching of the glass or quartz chamber may be anticipated and may indeed contaminate semiconductor device surfaces as a consequence of the chemical nature of the gases evolved from the chamber wall.
Other semiconductor related processing techniques employing RF plasma include the selective etching of metallic layers such as aluminum or other metal to form leads for semiconductor devices. Again patterned photoresists are employed to act as a barrier and the exposed aluminum or other metal is subjected to a plasma of an appropriate reactive gas or combination of gases usually containing a halogenated hydrocarbon gas. Still another example of RF plasma application to the semiconductor industry is in the preparation of metal or metal oxide on glass photomasks. Here again an imaged and developed photoresist barrier pattern over an underlying layer of chromium or iron oxide on glass permits gaseous plasma chemical attack by reactive species of gases such as chlorine, boron trichloride, and phosphorus pentachloride mixed with oxygen. Following this etching procedure the remaining photoresist can be chemically degraded and removed in an oxygen plasma or removed by means of a solvent outside the plasma reactor.
The object of this invention is to provide a plasma reactor device which will achieve uniformity of reactivity of the gaseous plasma over the entire target area. Another object is to provide distinct and independent means of defining and controlling the temperature of the plasma and that of the target or workpiece. Still another object is to provide a well defined and concentrated region of reactive gaseous plasma confined largely to the target areas and away from the walls of the reaction chamber. Further, an object of the present invention is to provide an effective plasma device in which the plasma is generated by a direct-current power supply which is less expensive and more stable than a radio frequency generated plasma and which does not require Federal Communication Commission approval. It is still a further object of this invention to provide controllable flexibility to plasma reactions and inter-reactions for the manufacture of semiconductors and other devices to be produced by photomechanical means which eliminates the need for exhaustive step-by-step tuning in the case of adventitious or deliberate change in any operating parameter.
Other objects, design characteristics and advantages will be obvious from description, preferred embodiments and accompanying drawings which follow.